1. Field of the Invention
The present invention relates generally to electroacoustic transducers, and more particularly to cylindrical transducers, actuated by piezoelectric or magnetic means, that can approximate monopole, dipole, or quadrupole acoustic radiators.
2. Description of the Related Art
Electroacoustic transducers provide a means for either generating and detecting acoustic vibrations in various sound propagation media. There are many types of acoustic transducers, with a common type being cylindrical transducers, which are characterized by a cylindrical shell that radiates or receives acoustic vibrations.
A further distinction among cylindrical transducers is the cylindrical "bender" transducer characterized by a flexural motion, or "bending", of the cylinder walls when the transducer is excited. Although cylindrical transducers, including cylindrical bender transducers, have been used to generate and receive acoustic waves in a variety of applications, their use has been characteristically limited to axi-symmetrical motions of the cylindrical structure of the transducers.
One form of cylindrical bender transducer used to generate and detect acoustic waves in geophysical applications is described in U.S. Pat. No. 4,525,646 to Shirley, et al. This transducer operates as a "self actuating" device, with the cylindrical shell being made of at least one piezoelectric layer. The active element of the transducer is the composite cylinder itself. All that is needed is the application of pressure waves or electrical energization for the transducer to operate as a detector or source. A limitation of this device is that it is directed toward a single motion of the cylinder walls, that motion being axi-symmetrical and therefore approximating a monopole source.
Geophysical exploration has long involved the use of both single borehole and hole-to-hole seismic measurements to determine characteristics of the surrounding geological environment. The seismic waves, which may be also referred to as acoustic or sonic waves, may be either compressional or shear, as well as other types of waves.
Various means have been used for generating and detecting the seismic waves. Most of the prior art involves the use of compressional waves, conveying by the earth's crust and detected on seismographs to give information about rock structures through which they travel. Of particular interest are those transducers which are capable of generating or detecting sound waves under conditions of deep borehole hydrostatic pressure and high temperatures. In the past, acoustic transducers have had difficulty meeting such rigorous environmental demands. Another requirement of such transducers, especially for hole-to-hole applications, is high power and high detection sensitivity over the frequency range of several hundred to several thousand hertz. Prior transducers have had difficulty meeting these specifications. Finally, transducers capable of preferentially generating shear waves has been a limitation in the prior art.